Round debug float output to 4 decimal places

Summary:
Float values in debug layout metrics output (used by Fantom test assertions) are
subject to floating-point precision differences across platforms and compilers
(e.g., `66.6667` vs `66.66666412353516`), causing flaky test comparisons.

This changes `toString(const double&)` in `DebugStringConvertible.cpp` to format
floats with 4 decimal places using `snprintf("%.4f")` and strip trailing zeros,
replacing `double-conversion`'s `ToShortest()`. This is debug-only code (guarded
by `#if RN_DEBUG_STRING_CONVERTIBLE`) so the precision change is safe for all
consumers.

The `double-conversion` dependency is also removed from the BUCK file since it is
no longer used.

Output examples:
- `0.0` → `"0"`
- `100.0` → `"100"`
- `66.66666412353516` → `"66.6667"`
- `1.5` → `"1.5"`
- `33.333333` → `"33.3333"`

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D95041966

Author: NickGerleman

packages/react-native/ReactCommon/react/renderer/debug/DebugStringConvertible.cpp

@@ -11,8 +11,6 @@
 #include <cinttypes>
 #include <cstdio>
 
-#include <double-conversion/double-conversion.h>
-
 namespace facebook::react {
 
 #if RN_DEBUG_STRING_CONVERTIBLE
@@ -129,20 +127,20 @@ SharedDebugStringConvertibleList DebugStringConvertible::getDebugProps() const {
  * `toString`-family implementation.
  */
 std::string toString(const double& value) {
-  // Format taken from folly's toString
-  static double_conversion::DoubleToStringConverter conv(
-      0,
-      "Infinity",
-      "NaN",
-      'E',
-      -6, // detail::kConvMaxDecimalInShortestLow,
-      21, // detail::kConvMaxDecimalInShortestHigh,
-      6, // max leading padding zeros
-      1); // max trailing padding zeros
-  std::array<char, 256> buffer{};
-  double_conversion::StringBuilder builder(buffer.data(), buffer.size());
-  conv.ToShortest(value, &builder);
-  return builder.Finalize();
+  std::array<char, 64> buffer{};
+  std::snprintf(buffer.data(), buffer.size(), "%.4f", value);
+  std::string result(buffer.data());
+
+  // Strip anything that ends up being an integer after reducing precision
+  if (auto dotPos = result.find('.'); dotPos != std::string::npos) {
+    auto lastNonZero = result.find_last_not_of('0');
+    if (lastNonZero == dotPos) {
+      result.erase(dotPos);
+    } else {
+      result.erase(lastNonZero + 1);
+    }
+  }
+  return result;
 }
 
 std::string toString(const void* value) {